Indium tin-oxide (ITO) is traditionally widely used as a transparent conductor in transparent electrodes in science and research community, but it also has well drawbacks in large scale manufacturing processes. First, in order to make electrodes, ITO is vacuum deposited onto substrates, and the vacuum deposition process is expensive and low throughput. Second, in most of applications, 150 nm or thicker of ITO is needed to ensure electrical performance, but at such thicknesses, ITO films become brittle making them not feasible for applications requiring large areas or flexible substrates. Third, to achieve good conductivity and clarity, ITO films need to be annealed at high temperatures, preferably over 200 C., thus limiting its application on high temperature resistant substrates such as glass. Due to the low softening point of polymers, most polymer based ITO films cannot withstand the annealing temperatures required for achieving the high conductivity and transparency at the same time. Therefore as electro-optical applications expand to more novel and exotic functionalities, such as 3-dimentional displays and solar cells, there is an increasing demand to invent alternative transparent electrodes with better than or comparable optical and electrical performance of ITO but suitable for large area flexible substrate and can be manufactured in an inexpensive high through manner.
Transparent conductive electrodes comprising printable metal nanowires have been successfully demonstrated as alternatives to be manufactured at low cost and on a large scale and with excellent performance including conductivity and transparency.
However the networked metal nanowires are not like the ITO films, having uniform conductivity across the entire film. The electrode having a plurality of metal nanowires, have areas containing metal nanowires laying on top of each other or crossing over. Research has found that reducing the metal nanowire junctions can significant reduce the sheet resistance of the conductive film.
Normally, when two nanowires stack together, it results in an intersection, having a height equal to the combined heights, i.e. diameters, of the two nanowires. For example, a conductive metal nanowire network comprises a first metal nanowire, having a diameter of d1, and a second metal nanowire, having a diameter of d2, and in the metal nanowire network, the first and second metal nanowire cross over to form a junction, then the junction height (J12) equals to d1+d2. FIG. 5 shows another example, a conductive electrode comprises a plurality of metal nanowires, the networked metal nanowires have a first metal nanowire with a diameter of d1, a second metal nanowire with a diameter of d2, and a third metal nanowire with a diameter of d3. In the metal nanowire network, the first, second and third metal nanowire cross over to form a junction, then the junction height J13 equals to the total height (i.e. diameter) of each metal nanowire, which is J13=d1+d2+d3. In FIG. 1, the first, second and third metal nanowire all have the same diameter (d1=d2=d3=d) and the junction height J13 equals to 3 d.
Research has found that high temperature annealing alone is not effective in melting the metal nanowire junction in order to reduce the sheet resistance. For example, anneal the dry film at a process condition 150-200 C., does not change the junction that has been formed, the sheet resistance of conductive film remains as high as over 1000 Ohms.
Approaches that have proven to be useful to change the nanowire junction is either to glue two wires together with a conductive polymer, as what has been taught in the art of carbon nanotubes, or using a high pressure press to flattened the junctions, as taught by US Publication 20110285019 and U.S. Pat. No. 8,049,333 in Cambrios patents. In US Publication 20110285019 and U.S. Pat. No. 8,049,333, external macroscopic force such as high pressure is used to flatten the junction to achieve the reduction in sheet resistance, in addition to high temperature annealing. However, the process introduces defects. Because nanowires are susceptible to damages, including physical deformation and/or thermal oxidation under high temperature and high-pressure process. Also the process using external force pressing the nanowires together is applied to the entire film, not only to the metal nanowire junction. Given the tiny dimension of nanowires, it requires very smooth and flat substrate surface to ensure the applied forces act on the junction. Otherwise, it is very likely that the nanowire length besides the junction is also pressed to be deformed or flattened, causing unnecessary stability issues.
In view of the foregoing, a better method to connect nanowires at the cross over points is needed.
The present invention discloses an improved way to integrate nanowires at cross points to form merged junctions, in order to achieve low sheet resistance of a transparent conductive electrode. The method disclosed herein does not require high temperature, high pressure, and does not result in deformed metal nanowires.